Soluble carbon nanotubes

ABSTRACT

The present invention relates to a method of solubilizing carbon nanotubes, to carbon nanotubes produced thereby and to uses of said carbon nanotubes.

Carbon nanotubes (CNTs) have recently attracted considerable attentiondue to their unique electronic, mechanical and structural properties.Carbon nanotubes have been shown to be electrically conducting at thesame time as having high tensile strength and elasticity, the ability toabsorb gas molecules as nanocapillaries, the potential of furtherchemical functionalization, and chemical and thermostability. Thesequalities make carbon nanotubes prime candidates for use innanomolecular and/or electronic devices.

Carbon nanotubes can be synthesized by a range of methods of whichinvolve the vaporization of elementary carbon by various means. Thefirst synthesis reported took place by the discharge of an electric arcof graphite in the presence of metal catalysts (e. g. Fe, Co, Ni).Alternative routes are based on the laser vaporization ofgraphite-Ni—Co-mixtures or chemical vapor deposition wherein variouscarbon sources can be used. At present milligram to gram quantities cancurrently be manufactured by using such routes, within a matter ofhours. The material, thus produced, however, has a substantial amount ofcontaminants and, in addition, side-wall defects. Carbon nanotubes whichresult directly from such synthesis methods, without having been furthermodified, are commonly referred to as “as-prepared carbon nanotubes”. Inorder to remove the contaminants, mainly oxidative treatments have beenimployed.

As-prepared carbon nanotubes (CNTs) that are produced in bulk quantities(usually via chemical vapor deposition, electric arc discharge, laserablation (also known as pulsed laser vaporization), or gas-phasecatalytic growth) are generally contaminated with large amounts ofimpurities (typically 5-50 wt %). The impurities include amorphouscarbon, graphite encapsulated catalytic metal particles, graphiticmaterial, and fullerenes. The most common methods for purifying the CNTsinvolve either liquid-phase or gas-phase oxidation processes, which maybe accompanied by solvent extraction, ultra-sonication, centrifugation,filtration, chromatography, and/or microwave exposure. The liquid-phaseoxidation processes generally make use of strong adds (primarily HNO₃,H₂SO₄, and/or HCl), sometimes with additional oxidants (e.g., H₂O₂,KMnO₄, etc.). Likewise, gas-phase oxidation is sometimes used inconjunction with liquid-phase add treatment, the latter being necessaryfor dissolution and removal of metal contaminants. Various reactionparameters such as concentration, temperature, and time have beenemployed, some representative examples of which are provided in Table 1.

TABLE 1 Published methods for purifying single-walled CNTs (processesconducted at room temperature unless otherwise noted; aqueous solutionsunless otherwise noted; filtration/rinsing and drying steps are omittedin most cases). CNT production method Purification steps ReferenceElectric arc 1. Reflux in H,O, 12 h Tohji et al. (1997)¹ 2. Extract withtoluene 4. Bake in air at 470° C., 20 min 5. Extract with 6 M HCI Laserablation 1. Extract with CS₂ Bandow et al. (1997)² 2. Sonicate in 0.1%surfactant solution, 2 h 4. Microfiltration (3 cycles) 5. Soak inethanol (to remove surfactant) Laser ablation 1. Reflux in concentratedHNO₃, 4 h Dujardin et al. (1998)³ 2. Centrifugation, H₂0 wash Laserablation 1. Reflux in 2.6 M HNO₃, 45 h Liu et al. (1998)⁴ 2.Centrifugation, H₂0 wash 3. Tangential flow filtration 4. Sonicate in3:1 H₂S0₄/HNO₃, 24 h 5. Treat with 4:1 H₂S0₄/30% H₂0₂ at 70° C., 0.5 hLaser ablation 1. Treat with Cl₂ + H₂O at 500° C., 6 h Zimmerman et al.Electric arc 2. Sonicate in 1:1 DMF: 0.6 M HCI (2000)⁵ 3. Sonicate inDMF Laser ablation 1. Reflux in 20% H₂0₂, 12 h Tang et al. (2000)⁶ 2.Extract with CS₂ and CH₃OH Electric arc 1. Bake in air at 300° C., 24 hRao and Govindaraj 2. Treat with conc. HNO₃ at 60° C. for (2001)⁷ 12 h3. Sonicate in ethanol and filter (0.3 pm) Laser ablation 1. Sonicate in1:1 HF/HNO₃ + surfactant, Chattopadhyay et al. 5 h. (2002)⁸ 2. Rinsewith 0.01 M NaOH Electric arc 1. Treat with microwaves (150 W) atHarutyunyan et al. 500° C., 20 min (2002)⁹ 2. Reflux in 4MH < 1.6 hElectric arc 1. Reflux in 2.8 M HNO₃, 6 h Kajiura et al. (2002)¹⁰ 2. Dryat 100° C., ≧10 h 4. Bake in air at 550° C., 10 min 5. Bake in vacuum at1600° C., 3 h

The more oxidative processes probably result in opening of the ends andenlarging the sidewall defects. As a result, the ends and surfaces ofthe CNTs can become covered with oxygen-containing groups such ascarboxylic acid, ether, phenolic, and quinone groups. Heating in avacuum to 230-330° C. thermally destroys carboxylic acid groups onsingle walled CNTs, while heating to 800° C. destroys ether and quinonegroups. Rao and Govindaraj subjected their purified CNTs toheat-treatment at 400° C. for 0.5 h to remove acid sites on the surfaceof the tubes and open them. (Rao et al. (2001) Proc. Indian Acad. Sci.(Chem. Sci.) 113, 375). Cai et al. ((2002) Chem. Mater. 14, 4235). foundthat oxygenated functional groups can be removed from single-walled CNTsby heating at 600-800° C. for 5 h. Thermal annealing in vacuum at1000-1200° C. is expected to cause the open ends to close tohemifullerene end caps. (Liu et al. (1998) Science 280, 1253.)

At present there is one drawback associated with carbon nanotubes whichprevents them from being properly processed and further manipulated,which is that carbon nanotubes, inherently, are largely insoluble inmost common solvents.

This intrinsic insolubility arises from strong van der Waals and π-πstacking interactions between individual CNTs, which causes them toorient parallel to one another in a close-packed triangular lattice.These structures are referred to as “ropes” or “bundles.” CNTs that havebeen subjected to oxidative treatment may have an even greater tendencyto form bundles due to carboxyl related secondary bonding forces.Methods that have been developed for enhancing the solubility of CNTsinclude cutting them into shorter pieces and functionalization. Itshould be noted that stable dispersions rather than true solutions ofCNTs are obtained by some of these methods.

Cutting or shortening of CNTs can be achieved by either mechanical orchemical means. The mechanical methods used include ultrasonication,ball milling, and abrasion. Liu et al. ((1998) Science 280, 1253),combined ultrasonication with chemical “etching” for cuttingsingle-walled CNTs.

Hirsch ((2002) Angew. Chem. Int. Ed. 41, 1853) recently reviewed thefunctionalization of single walled CNTs. There is special interest inwater-soluble CNTs for biological applications.

Functionalization of CNTs can be classified as either covalent ornon-covalent. Several covalent processes utilize the carboxylic acid(—COOH) group that is generated during oxidative purification forattaching organic amines or alcohols to the CNTs via amide or ester bondformation, respectively. The organic compounds attached includedpolymers and dendrons. Metal complexes can be attached to single-walledCNTs by coordinative bonding of the metal center to the oxygenatedcarbon groups. Other covalent modifications described in the literatureinclude reversible sidewall-alkylation of fluorinated CNTs (Boul et al.(1999) Chem. Phys. Lett. 310, 367), reactions with aryl diazoniumcompounds (Bahr et al. (2001) J. Am. Chem. Soc. 123, 6536; Bahr et al.(2001) Chem. Mater. 13, 3823; Kooi et al. (2002) Angew. Chem. Int. Ed.41, 1353), ultrasonically-induced reactions with monochlorobenzene andpoly(methyl methacrylate) (Koshio et al. (2001) Nano Lett. 1, 361),addition reactions of nitrenes and nucleophilic carbenes (Holzinger etal. (2001) Angew. Chem. Int. Ed. 40, 4002), and addition reactions ofazomethine ylides (Georgakilas et al. (2002) J. Am. Chem. Soc. 124,760).

Non-covalent processes for functionalization of CNTs generally involveadsorption of molecules to the sidewalls. Surfactants used for thispurpose include sodium dodecyl sulfate (Duesberg et al. (1998) Appl.Phys. A 67, 117; Doom et al. (2002) J. Am. Chem. Soc. 124, 3169) andTriton. The alcohol moiety on Triton surfactants can be used forsubsequent covalent chemical modification (Shim et al. (2002) Nano Lett.2, 285). Polymers can adsorb by a process called “polymer wrapping”(Curran et al. (1998) Adv. Mater. 10, 1091; Tang et al. (1999)Macromolecules 32, 2569; Coleman et al. (2000) Adv. Mater. 12, 213;O'Connell et al. (2001) Chem. Phys. Lett. 342, 265; Bandyopadhyaya etal. (2002) Nano Lett. 2, 25; Star et al. (2002) Angew. Chem. Int. Ed.41, 2508; Chen et al. (2002) J. Am. Chem. Soc. 124, 9034; Star et al.(2002) Macromolecules 35, 7516). Salt formation between organic aminesand the carboxylic acid groups on oxidized CNTs is another non-covalentmethod for making CNTs soluble (Hamon et al. (1999) Adv. Mater. 11, 834;Chen et al. (2001) J. Phys. Chem. 105, 2525; Chattopadhyay et al. (2002)J. Am. Chem. Soc. 124, 728; Kahn et al. (In press) Nano Lett). Lastly,adsorption of pyrene derivatives to the sidewalls of CNTs via π-stackinginteractions was used for functionalization. The succinimidyl ester of1-pyrenebutanoic acid served as a chemically reactive site for formingattaching proteins to the CNTs via amide bond formation (Chen et al.(2001) J. Am. Chem. Soc. 123, 3838).

Despite the above-mentioned cutting and functionalization processes, thecarbon nanotubes still exist predominantly in the form of bundles, i. e.agglomerates of carbon nanotubes in parallel. In the past, severalattempts have been made to separate these bundles into single (in thesense “non-bundled”) carbon nanotubes via a process referred to as“exfoliation”.

Treatment of bundles of single-walled CNTs with 70% nitric acid at 25°C. for 2 hours resulted in an expansion of the inter-nanotube spacingand increase in the amount of hydrogen in the material (Bower et al.(1998) Chem. Phys. Lett. 288, 481). These changes, which could bereversed by heating to 230° C. under vacuum for 12 hours, indicatedreversible intercalation of the bundles with HNO₃ molecules. Treatmentwith the acid for 12 hours resulted in a change in structure andmorphology of the CNTs that was not reversed by heating under vacuum.Liu et al. ((1998) Science 280, 1253) chose a 3:1 concentratedH₂SO₄:HNO₃ mixture for the oxidizing acid during cutting of CNTs becauseit was known to intercalate and exfoliate graphite.

Exfoliation is also believed to be a key feature of the mechanicalcutting process reported by Chen et al. ((2001) J. Am Chem. Soc. 123,6201) which involved grinding single-walled CNTs in soft organicmaterials such as γ- and β-cyclodextrins. According to the authors, theexcellent dispersion of the CNTs by the cyclodextrins, together withpartial exfoliation, made the grinding forces sufficiently strong toinduce local conformational strains on the nanotubes, which eventuallyresulted in cutting, most likely at their defective sites.

Filtration is a method of collecting and washing carbon nanotubesfollowing wet chemical treatments that is widely used in the art.Membrane filters, in particular, are generally used. Some examples are:polycarbonate track-etched filter membrane with 0.8 μm pore size(Shelimov et al. (1998) Chem. Phys. Lett. 282, 429), PTFE filtermembrane with 0.2 μm pore size (Mickelson et al. (1999) J. Phys. Chem. B103, 4318; Star et al. (2002) Angew. Chem. Int. Ed. 41, 2508)), andpolycarbonate filter membrane with 3 μm pore size (Yu and Brus (2001) J.Phys. Chem. B105, 1123).

The methods presently known for solubilization ar generally verytime-consuming and/or require special equipment or chemicals and/or arenot easily scaled up to large quantity. Furthermore, they may requirethe use of CNTs that have been shortened. Accordingly there is a need inthe art for simpler solubilization methods that provide soluble carbonnanotubes capable of dissolving in a variety of solvents, includingwater. Furthermore there is a need in the art for providing a method ofsolubilizing carbon nanotubes that have not been shortened and/orprovide functional groups for subsequent further chemical modification.Also there is a need in the art for a method of solubilizing carbonnanotubes that are applicable to both multi-walled and single-wailedcarbon nanotubes that also allow the solubilization of gram quantitiesof carbon nanotubes.

All these objects are solved by a method of solubilizing carbonnanotubes comprising the steps:

-   -   a) providing, in any order:        -   carbon nanotubes, and at least one type of monomer molecules            capable of undergoing a polymerization reaction or a            precursor of the at least one type of monomer molecules;    -   b) mixing together the nano tubes with the monomer molecules or        their precursor;    -   c) initiating a polymerization reaction of the monomer molecules        to yield modified carbon nanotubes, wherein the carbon nanotubes        have functional groups on their surface and/or ends, and wherein        said polymerization reaction occurs at said functional groups.

“Solubilizing” is not restricted to a particular kind of solvent.Preferably it means solubilizing in aqueous solution or an alcoholicsolution, more preferably a methanolic solution.

In one embodiment, the functional groups are oxygenated functionalgroups selected from the group comprising C—O species (alcohol, phenol,ether, epoxide), C═O species (aldehyde, ketone, quinone), and O—C═Ospecies (carboxylic acid, ester, anhydride, lactone, pyrone).

It is preferred that the carbon nanotubes are single-walled ormulti-walled nanotubes.

In one embodiment approximately 1 per 200 to 1 per 10 carbon atoms ofthe carbon nanotubes, preferably approximately 1 per 150 to 1 per 20carbon atoms of the carbon nanotubes, more preferably approximately 1per 100 carbon atoms of the carbon nanotubes are in an oxidized state.

It is preferred that the monomer molecules are isocyanic acid and/orcyanate ion.

In one embodiment the precursor of the at least one type of monomermolecules is urea and/or its derivatives.

In another embodiment, the precursor of the at least one type of monomermolecules is selected from the group comprising cyanuric acid, cyanuricchloride, isocyanuric acid and trichloroisocyanic acid.

In yet another embodiment, the precursor is a cyanate salt, such as thecyanate salt of an alkali metal or a quaternary ammonium cyanate.

It is preferred that the method according to the present inventioncomprises the additional step(s):

-   -   ba) heating the mixture, or, alternatively,    -   bb) acidifying the mixture or,    -   both steps ba) and bb), wherein, preferably the heating and/or        acidifying is such, that the precursor of the at least one type        of monomer molecules is induced to form said monomer molecules.

Preferably, the at least one type of monomer molecules or the precursorof the at least one type of monomer molecules is provided in a solvent.

In one embodiment the carbon nanotubes are provided in a solvent.

Preferably, the solvent of the monomer or its precursor is the same asthe solvent of the carbon nanotubes, or the solvents are different.

In one embodiment, the solvent(s) can be heated to a temperature closeto or above the melting point of the monomer or its precursor, withoutdecomposing.

In one embodiment, before, during or after the polymerization reaction,at least one aldehyde is added to the mixture, wherein, preferably, theat least one aldehyde is selected from the group comprisingacetaldehyde, benzaldehyde, carboxybenzaldehyde, cinnamaldehyde,chlorobenzaldehyde, ferrocene carboxaldehyde, formaldehyde, furfural,glutaraldehyde, paraformaldehyde, polyhydroxyaldehyde, propionaldehyde,pynidine aldehyde, salicylaldehyde and valeraldehyde.

A polyhydroxyaldehyde is a class of carbohydrate including aldoses. Analdose is a monosaccharide sugar that contains the aldehyde group(—CHO). An aldose can further be classified as aldotriose, aldotetrose,aldopentose, and aldohexose, depending on the number of carbon atoms inthe sugar. Examples of these are glyceraldehyde (an aldotriose),erythrose (an aldotetrose), ribose (an aldopentose), and glucose (analdohexose). Aldopentose and aldohexose compounds exist in aqueoussolution in equilibrium with their five or six member ring hemiacetalforms. Certain di-, tri-, and polysaccharides that contain aldosecomponents are also polyhydroxyaldehyde compounds according to thepresent invention. Examples of disaccharides that arepolyhydroxyaldehydes are maltose and lactose.

Preferably the benzaldehyde is substituted with at least oneelectron-donating group, selected from —NHR, —NRR′, —OH, —OR, —C₆H₅,—CH₃, —CH₂R, —CHR₂ and CR₃, wherein R and R′ represent linear orbranched C₁-C₁₂ alkyl groups, C₃-C₈ cycloalkyl groups, C₆-C₁₂ aralkylgroups, C₆-C₁₂ aryl groups, poly(ethylene oxide), poly(propylene oxide),and poly(ethylene oxide)-co-poly(propylene oxide)block co-polymers.

In one embodiment, the at least one electron-donating group onbenzaldehyde is in the para-position.

Preferably, the at least one electron-donating group on benzaldehyde is—OH or —OR, wherein R represents a linear or branched C₁-C₁₂ alkylgroup, a C₃-C₈ cycloalkyl group, a C₆-C₁₂ aralkyl group, a C₆-C₁₂ arylgroup, poly(ethylene oxide), poly(propylene oxide), or poly(ethyleneoxide)-co-poly(propylene oxide)block co-polymer.

More preferably, the at least one aldehyde is selected from the groupcomprising p-anisaldehyde, 4-propoxybenzaldehyde and4-(hexyloxy)benzaldehyde.

If the aldehyde is added before and/or during polymerization, in oneembodiment of the method according to the present invention, it shouldhave a boiling point greater than approximately 100° C. Paraformaldehydeis a non-volatile polymeric form of formaldehyde that depolymerizes toformaldehyde. Benzaldehyde and glutaraldehyde are common aldehydes withhigh boiling points (>170° C.).

The criterion for the boiling point of the aldehyde in this particularembodiment, is that the aldehyde can be present during thepolymerization long enough to react without evaporating completely.

In one embodiment, after step c), non-reacted monomer and/or precursoris removed from the reaction, wherein, preferably, the removal occurs bya salt precipitation step, and/or by centrifugation or filtration, eachof both possibilities optionally followed by washing, and/or the removalfurther occurring by size separation and/or adsorption, and/or byenzymatic degradation, and/or by selective burning, and/or by plasmatreatment, and wherein, even more preferably, the salt precipitationoccurs by addition of a salt, preferably a perchlorate salt, inparticular sodium perchlorate, the filtration occurs using a membranefilter having a pore size smaller than 1 μm but larger than 0.01 μm, thesize separation and/or adsorption occurs by gel-filtration, preferablyover a dextran-based material, more preferably Sephadex, and theenzymatic degradation occurs by means of urease. In one embodiment, theprecipitation selectively precipitates modified CNTs.

Selective burning is based on the principle that most organics will bumin air under conditions where CNTs are stable. Therefore heating toappropriate temperatures for appropriate times will result in the CNTsremaining while other organics have simply burned away. Reasonabletemperature and time ranges are 300° C. to 550° C. and 10 minutes to 24hours (shorter times when higher temperatures are used).

Similarly plasma treatment can be used for selective removal of organicson CNTs, and the conditions are similar to ones that are used to cleansilicon substrates (these will depend on the type of plasma generator):Exemplary conditions are

Oxygen plasma treatment at room temperature

Applied radio frequency (RF): 13.56 MHz

RF power: 10-100 W

O₂ pressure: 0.1-1 mbar

Time: 10 seconds to 5 minutes

In one embodiment, after polymerization an amine-reactive compound, suchas carboxylic acid anhydride, is added and reacted with the modifiedcarbon nanotubes.

It is preferred that the modified carbon nanotubes are dissolved inaqueous solution or in alcoholic solution, preferably methanolicsolution.

The object of the present invention are also solved by a method ofsolubilizing carbon nanotubes, comprising the steps:

-   -   a) providing, in any order:        -   carbon nanotubes, and        -   urea;    -   b) mixing together the nanotubes and the urea;    -   c) heating the mixture of b).

Preferably, the carbon nanotubes are as defined above.

In one embodiment, the heating is above the melting temperature of urea,wherein, preferably, the heating is in the range of approximately 130°C.-180° C., more preferably approximately 150° C.-170° C., even morepreferably approximately 150-160° C., most preferably approximately 150°C.

In one embodiment, the heating is for approximately 1-60 min, preferablyapproximately 3-20 min, more preferably approximately 5-15 min, mostpreferably approximately 10 minutes.

In one embodiment, the at least one aldehyde is added to the mixture ofb). Preferably it is added to the mixture of b) before or during stepc). In one embodiment the at least one aldehyde is added during step c),preferably 1-5 minutes after step c) has been initiated, most preferably1-3 minutes after step c) has been initiated.

In one embodiment, the at least one aldehyde is added to the mixture ofb) during step c), preferably after the urea is completely molten.

In one embodiment, for the purpose of adding the at least one aldehydeduring step c), as outlined before, the heating is interrupted whenadding the aldehyde, and is thereafter resumed. In one embodiment,wherein for the purpose of adding the at least one aldehyde during stepc), the heating is interrupted when adding the aldehyde, and isthereafter resumed, the total duration of heating, as recited above(“the heating is for approximately 1-60 min. preferably approximately3-20 min, more preferably approximately 5-15 min, most preferablyapproximately 10 min”), this total duration includes the time of addingthe aldehyde. In another embodiment, the total duration of heating isfor approximately 1-60 min, preferably approximately 3-20 min, morepreferably approximately 5-15 min, most preferably approximately 10 min,irrespective of any addition of aldehyde.

In one embodiment, the at least one aldehyde is selected from the groupcomprising acetaldehyde, benzaldehyde, carboxybenzaldehyde,cinnamaldehyde, chlorobenzaldehyde, ferrocene carboxaldehyde,formaldehyde, feral, glutaraldehyde, paraformaldehyde,polyhydroxyaldehyde, propionaldehyde, pyridine aldehyde, salicylaldehydeand valeraldehyde.

A polyhydroxyaldehyde is a class of carbohydrate including aldoses. Analdose is a monosaccharide sugar that contains the aldehyde group(—CHO). An aldose can further be classified as aldotriose, aldotetrose,aldopentose, and aldohexose, depending on the number of carbon atoms inthe sugar. Examples of these are glyceraldehyde (an aldotriose),erythrose (an aldotetrose), ribose (an aldopentose), and glucose (analdohexose). Aldopentose and aldohexose compounds exist in aqueoussolution in equilibrium with their five or six member ring hemiacetalforms. Certain di-, tri-, and polysaccharides that contain aldosecomponents are also polyhydroxyaldehyde compounds according to thepresent invention. Examples of disaccharides that arepolyhydroxyaldehydes are maltose and lactose.

Preferably the benzaldehyde is substituted with at least oneelectron-donating group, selected from —NHR, —NRR′, —OH, —OR, —C₆H₅,—CH₃, —CH₂R, —CHR₂ and CR₃, wherein R and R′ represent linear orbranched C₁-C₁₂ alkyl groups, C₃-C₈ cycloalkyl groups, C₆-C₁₂ aralkylgroups, C₆-C₁₂ aryl groups, poly(ethylene oxide), poly(propylene oxide),and poly(ethylene oxide)-co-poly(propylene oxide)block co-polymers.

In one embodiment, the at least one electron-donating group onbenzaldehyde is in the para-position.

Preferably, the at least one electron-donating group on benzaldehyde is—OH or —OR, wherein R represents a linear or branched C₁-C₁₂ alkylgroup, a C₃-C₈ cycloalkyl group, a C₆-C₁₂ aralkyl group, a C₆-C₁₂ arylgroup, poly(ethylene oxide), poly(propylene oxide), or poly(ethyleneoxide)co-poly(propylene oxide)block co-polymer.

More preferably, the at least one aldehyde is selected from the groupcomprising p-anisaldehyde, 4-propoxybenzaldehyde and4-(hexyloxy)benzaldehyde.

In one embodiment, the product of step c) is dissolved in aqueoussolution or alcoholic, preferably methanolic, solution or a mixture ofan aqueous and an alcoholic solution and subjected to a saltprecipitation step, and/or centrifugation or filtration, each of bothpossibilities optionally followed by washing, and/or subjected tosize-separation and/or adsorption, and/or enzymatic degradation, and/orselective burning, and/or plasma treatment.

In one embodiment, the salt precipitation occurs by addition of a salt,preferably a perchlorate salt, in particular sodium perchlorate, thefiltration occurs using a membrane filter having a pore size smallerthan 1 μm but larger than 0.01 μm, the size separation and/or adsorptionoccurs by gelfiltration, preferably over a dextran-based material, morepreferably, Sephadex, and the enzymatic degradation occurs by means ofurease. In one embodiment, the precipitation selectively precipitatesmodified CNTs.

Preferably the product of step c) is dissolved in an aqueous solution oralcoholic, preferably methanolic solution or a mixture of an aqueous andan alcoholic solution and is subjected to an evaporation step.

The objects of the present invention are also solved by a carbonnanotube, produced by the method according to the present invention.

Preferably the nanotube is non-bundled.

In one embodiment, the carbon nanotube is decorated in a pearlchain-like manner with discrete bodies, when viewed by AFM.

In one embodiment these discrete bodies are polymeric, “polymeric”, inthis context meaning “comprising polymers”.

Preferably, the carbon nanotube has one or more physical characteristicsselected from the group comprising:

-   -   a solubility in water or aqueous solution of up to approximately        10 g/l and/or solubility in methanol or methanolic solution of        up to approximately 1 g/l.    -   an absorption maximum between 210 nm and 250 nm,    -   an emission maximum between 500 nm and 550 nm, when excited with        UV or blue light (330 nm-430 nm), and    -   one or several or all of the following absorption maxima in the        infrared in the following wavenumber regions:        -   3470-3490 cm⁻¹, 3420-3440 cm⁻¹, 3365-3385 cm⁻¹, 3330-3350            cm⁻¹, 3245-3265 cm⁻¹, 3210-3230 cm⁻¹, 1660-1680 cm⁻¹,            1610-1630 cm⁻¹, 1450-1470 cm⁻¹, 1330-1350 cm⁻¹ and 1095-1115            cm⁻¹.

It has to be said, though, that the inventors do not wish to be limitedto the notion that these physical properties are intrinsic to thenanotubes themselves. These characteristics may also be due to thediscrete bodies attached to the nanotubes or due to the preparation ofthe nanotubes. This may apply, in particular, to the emission maximummentioned above.

The objects of the present invention are also solved by an associationof carbon nanotubes according to the present invention, wherein thecarbon nanotubes are non-bundled, but interconnected at their endsthrough said discrete bodies into branched structures, when viewed underAFM.

The objects of the present invention are also solved by a carbonnanotube or an association of carbon nanotubes, which has been producedby the method, in which method the product of step c) is dissolved inaqueous solution or alcoholic, preferably methanolic, solution or amixture of an aqueous and an alcoholic solution, and is subjected to anevaporation step.

Preferably, such a carbon nanotube or association of carbon nanotubes ispreferably at least partially crystalline.

In one embodiment such a carbon nanotube or association of carbonnanotube has one or more physical characteristics, selected from thegroup comprising:

-   -   one or several or all of the following absorption maxima in the        infrared in the following wavenumber regions:        -   3370-3390 cm⁻¹, 3205-3225 cm⁻¹, 3060-3080 cm⁻¹, 1700-1720            cm⁻¹, 1680-1700 cm⁻¹, 1655-1675 cm⁻¹, 1580-1600 cm⁻¹,            1500-1520 cm⁻¹, 1440-1460 cm⁻¹, 1395-1415 cm⁻¹, 1245-1265            cm⁻¹, 1155-1175 cm⁻¹, 1020-1040 cm⁻¹, 845-865 cm⁻¹, 795-815            cm⁻¹,    -   having both crystalline and amorphous parts when viewed under        AFM, TEM and/or SEM,    -   a solubility in water or aqueous solution of up to approximately        10 g/l and/or a solubility in methanol or methanolic solution of        up to approximately 1 g/l.

The objects of the present invention are also solved by the use of acarbon nanotube or of an association of carbon nanotubes according tothe present invention as outlined above in an electronic device, ananoelectrotic device, a memory element, a field emission device, asensor, an actuator, an electromechanical device, a composite material,a coating/paint/paste, a hydrogen storage device, a battery or fuelcell, a supercapacitor, a photoelectrochemical device, a photovoltaicdevice, an energy conversion device, a light emitting diode, a liquidcrystal display, a probe scanning probe microscopy, a non-linear opticaldevice or antenna, or a catalyst.

The objects of the present invention are also solved by the use of themethod according to the present invention for orienting and/or purifyingcarbon nanotubes, in particular single walled carbon nanotubes (SWNTs).

For purposes of orienting the carbon nanotubes, the method according tothe present invention is preferably performed on a surface, morepreferably on a polymer or silicon-based surface.

It should be understood that the mixing of the carbon nanotubes and theat least one type of monomer (or the precursor thereof, or urea etc.)can be achieved by various means, such as are well known to someoneskilled in the art. These include, but are not limited to grinding,milling, in particular ball-milling, sand-milling etc. Furthermore it isclear that the carbon nanotubes provided can be treated with othermethods that are commonly known to someone skilled in the art. Forexample they may, before or after the reaction of the present invention,be ultra-sonicated, they may be heated in a vacuum or under inertatmosphere or they may be further functionalized. If they are thermallyannealed (heated) and this takes place after the reaction of theinvention, this may remove the functionalization achieved by thereaction of the invention and even reverse the effects of a previousoxidative treatment. Such a subsequent thermal annealing treatment maybe necessary to restore the electronic properties of pristine carbonnanotubes. Temperatures in the range of approximately 200-1200° C. areeffective for this purposes. It is also clear that various means of sizeseparation can be used, so as to separate unreacted small molecule partsfrom the reacted portions. For example appropriate gel-filtrationmaterial, chosen by the experimenter depending on the desired sizeexclusion, can be used. Various grades of Sephadex gel-filtrationmaterial seem to be appropriate in a preferred embodiment, for exampleSephadex G-100 or G-10. However other types of Sephadex anddextran-based materials can be used instead, the only criterion beingthat an effective separation of small molecules from portions/compoundshaving undergone the reaction(s) of the present invention takes place.Furthermore, small molecules and non CNT-particles and/or non-modifiedCNTs may be removed by adsorption to a gel-filtration material, whichmaterial may be the same as defined above for the size-exclusion step.Furthermore, small molecules can also be removed by the use ofappropriate degradation enzymes, e.g. urease in the case of urea.Another way of separation is selective precipitation of (modified) CNTs,e. g. by means of a salt, preferably a perchlorate, in particular sodiumperchlorate. Furthermore filtration may be used as a-method ofcollecting and washing carbon nanotubes, following wet chemicaltreatments, as has already been outlined above.

As used herein, the term “precursor” is meant to encompass any chemicalentity that is capable of providing monomers for a subsequentpolymerization reaction. The term “monomer”, as used herein, is meant toencompass any chemical entity that is capable of reacting with likemolecules and thereby form a larger entity comprising more than one ofthe original monomers. It is clear that the term “monomer” alsoencompasses “oligomers” which are still capable of undergoing apolymerization reaction.

A reaction or polymerization is said to occur “at the functional groups”in the sense that the growing polymer chain or reaction product, at oneof its ends, is either covalently bonded to a functionalgroup/functional groups or non-covalently adsorbed thereto, or both, inthe sense that within one experimental set-up some growing polymerchains or reaction products are covalently bonded to functional groupand other polymer chains or reaction products are non-covalentlyadsorbed thereto. The term “occurs at the functional groups”, as, usedherein, is used interchangeably with “is initiated by the functionalgroups”. It can also mean that the functional groups serve as initiationsites for polymerization. In doing so, the functional groups may eitherserve directly as a site where polymerization occurs or they may firstbe converted to a functional group at which then the polymerizationoccurs. Preferably this conversion is due to the monomer moleculesand/or their precursor and/or a decomposition product of the monomer orits precursor. An example for the latter reaction can be seen from FIG.3 in “Reactions involving epoxide groups”, where ammonia, adecomposition product of urea, converts an epoxide to an amine and anOH-group where then polymerization takes place.

It should also be understood that the heating step according to thepresent invention can be performed by any conventional heating means, e.g. an oven, hot plate, oil bath or heat gun. In one embodiment it isperformed using a heat gun.

“Derivatives of urea” are e. g. nitrourea, urea hydrochloride, ureahydrogen peroxide, urea nitrate, and ammonium carbamate, all of whichcan act as sources of ultimately isocyanic acid.

It has surprisingly been found that performing a polymerization reactionon the carbon nanotubes, in effect, exfoliates the tubes and solubilizesthem such that afterwards they are present as single, non-bundledentities. The method according to the present invention, as opposed toprior-art-methods, yields non-bundled carbon nanotubes, i. e. tubeswhich are not in an association of essentially parallel CNTs that are incontact with each other. A preferred polymerization reaction that istaking place on the carbon nanotubes is the polymerization of isocyanicacid/cyanate as a decomposition product of urea, to polyisocyanateappendages. Urea has often been used as a source of isocyamic acid. Forsuch purposes, urea can either be used neat in the molten state (mp 133°C.), or dissolved in water or an organic solvent. Alternative exemplarysources of isocyanic acid include a) thermal depolymerization ofcyanuric acid or isocyanuric acid, b) hydrolysis of cyanuric chlorideand subsequent depolymerization of the resultant cyanuric acid, c)acidification of cyanate salts, d) hydrolysis of trichloroisocyanic acidand subsequent depolymerization of the resultant isocyanuric acid, ande) thermal decomposition of nitrourea (Davis et al. (1929) J. Am. Chem.Soc. 51, 1790). All these sources of isocyanic acid are to, beunderstood as “precursors of the at least one type of monomer” whereisocyanic acid or cyanate ion is the monomer in the present invention.

It has also surprisingly been found that a solubilization of nanotubescan be achieved by letting a urea melt react with the nanotubes,preferably prepurified ones. The yield of soluble nanotubes can be evenfurther improved by adding an aldehyde to the melt. The nanotubes thusproduced can form crystal-like structures when they have becomedissolved and the solvent becomes evaporated. In that case,crystal-like, birefringent structures are formed, presumably containingaligned nanotubes, which further assemble into more complex forms havingdimensions of up to ca. 100 μm.

In the following, the chemistry of urea and isocyanic acid/cyanate isfurther explained.

Despite its simple structure, the chemistry of urea (NH₂CONH₂) can bequite complex, mainly because of the extremely reactive nature of one ofits decomposition products, isocyanic add (HNCO). It is often assumedthat an equilibrium exists between urea and ammonium cyanate (H₄CNO):

NH₂CONH₂

NH₄CNO,  (1)

The synthesis of urea from ammonium cyanate by Wöhler was the firstexample of an “organic” compound being synthesized in the laboratoryfrom purely “inorganic” ones. Much of the current interest in thechemistry of urea stems from its possible role in pre-biotic evolution,as well as its importance as a fertilizer.

Since ammonium ion (NH₄ ⁺) is weakly acidic and cyanate ion (CNO⁻) isweakly basic, the equilibrium in equation (1) can be extended to includeammonia (NH₃) and cyanic acid (CNOH):

NH₄CNO

NH₃+CNOH  (2).

While cyanic acid is stable enough to be isolated, isocyanic add is thethermodynamically favored isomer under normal conditions.

CNOH

HNCO  (3)

Both isomers are very chemically reactive.

Isocyanic acid, like its organic isocyanate R—N═C═O) derivatives, canreact with compounds containing active hydrogen atoms, includingcarboxylic acids and phenols. Standard methods for purifying SWNTsinvolve oxidizing conditions that generate such oxygenated functionalgroups at their ends and at defect sites along their sidewalls. Based onthese considerations, the present inventors anticipated that molten ureacould react with acid-purified SWNTs, resulting in covalent attachmentof highly polar substituents derived from urea. For example, SWNT-COOHcould react with HNCO to form CO₂ plus the amide (SWNT-CONH₂). Thelatter could further react with HNCO to form the acylurea(SWNT-CONHCONH₂). Likewise, SWNT-OH could react with HNCO to form theurethane (SWNT-OCONH₂), which could further react to form theallophanate (SWNT-OCONHCONH₂). Oxygenated functional groups notcontaining active hydrogen atoms that may be present on purified SWNTs(e.g., anhydride, lactone, ketone, epoxide) could participate byreacting first with NH₃, which is also present in molten urea, andsubsequently with HNCO. These various addition reactions of HNCO may bereversible. The present inventors further investigated the possibilityof tuning the solubility of SWNTs modified by molten urea byincorporating para-substituted alkoxy benzaldehyde derivatives, inparticular p-anisaldehyde, 4-propoxybenzaldehyde, or4-(hexyloxy)benzaldehyde into the process.

In summary, isocyanates (RNCO), which are organic derivatives ofisocyanic acid, are very reactive and undergo a great many reactions,especially addition reactions with compounds containing active hydrogenand polymerization (self-addition). Several of these reactions arelisted below and are representative of the reactions of isocyanic acid(where R=H) as well.

-   -   1. Reaction with alcohols and phenols to form urethanes:

RNCO+HOR′→RNH—CO—OR′  (4)

-   -   2. Reaction with amines to form ureas:

RNCO+H₂NR′→RNH—CO—NHR′  (5)

-   -   3. Reaction with carboxylic acids to form amides:

RNCO+HOOCR′→RNH—COR′+CO₂  (6)

-   -   4. Reaction with urethanes to form allophanates:

RNCO+R′NH—CO—OR″→RNH—CO—NR′—CO—OR″  (7)

-   -   5. Reaction with ureas to form biurets:

RNCO+R′NH—CO—NHR″→RNH—CO—NR′—CO—NHR″  (8)

-   -   6. Reaction with amides to form acylureas:

RNCO+R′NH—COR″→RNH—CO—NR′—COR″  (9)

-   -   7. Dimerization to form uretidiones:

-   -   8. Trimerization to form isocyanurates:

-   -   9. Hydrolysis to form amines:

RNCO+H₂O→RNH₂+CO₂  (12)

Urea has often been used as a source of isocyanic acid for the abovereactions. For such purposes, urea can either be used neat in the moltenstate (mp 133° C.), or dissolved in water or an organic solvent.Alternative sources of isocyanic acid include thermal depolymerizationof cyanuric acid, isocyanuric acid, hydrolysis of cyanuric chloride andsubsequent depolymerization of the resultant cyanuric acid acidificationof cyanate salts, hydrolysis of trichloroisocyanic acid and subsequentdepolymerization of the resultant isocyanuric acid, and thermaldecomposition of nitrourea (Davis et al. 1929 J. Am. Chem. Soc. 51,1790). All these sources of isocyanic acid are to be understood as“precursors of the at least one type of monomer” where isocyanic acid orcyanate ion is the monomer in the present invention.

Formaldehyde (or paraformaldehyde) reacts with urea to form additionproducts with hydroxymethyl end groups or methylene bridges between ureaunits:

H₂NCONH₂+CH₂O→H₂NCONHCH₂OH  (13)

H₂NCONHCH₂OH+H₂NCONH₂→H₂NCONHCH₂NHCONH₂+H₂O  (14)

Such urea-formaldehyde condensation products are industrially importantin materials ranging from plastics and adhesives to fertilizers.

Furthermore, urea is able to form solid-state inclusion compounds (or“clathrates”) with long-chained hydrocarbons such as n-alkanes. The hoststructure consists of continuous one-dimensional channels (ca 0.6 nmdiameter) constructed from an essentially infinite three-dimensionalhydrogen-bonded network of urea molecules. (Steed, J. W.; Atwood, J. L(2000) Supramolecular Chemistry; John Wiley & Sons, Ltd, Chichester, pp.272-277) The urea channel structure is only stable when occupied. Ureainclusion compounds based on a layered structure (Lee et al. (2001) J.Am. Chem. Soc. 123, 12684) as well as inclusion compounds involving bothurea and one of its decomposition products (Mak et al. (1995) J. AmChem. Soc. 117, 11995) are also known.

The process that the present inventors developed involves intimatemixing e.g. grinding pre-purified SWNTs with urea before heating. Thepre-purified SWNTs used in the experiments are RFP-SWNT from CarbonSolutions, Inc. (Riverside, Calif.). RFP-SWNT consists of nanotubesprepared by the electric arc method that are acid-purified andsubsequently processed to reduce functionality. According to thesupplier, the RFP-SWNT used in the experiments performed in thisinvention have a relative purity of 40%, as evaluated according to M. E.Itkis et al., Nano Lett. 3, 309 (2003). The mixing process mayfacilitate penetration of urea into the nanotube bundles as well as toincrease the density of functional groups on the nanotubes by breakingthem mechanically. AFM images of the starting RFP-SWNT and UA-SWNTs showmostly bundled nanotubes in both cases. In general, the bundles inRFP-SWNT samples are longer (0.5-10 μm) than those in UA-SWNT samples(0.5-1.5 μm). However, this comparison is purely qualitative, since itis usually not possible to observe both ends of individual nanotubeswithin the bundles. Individual UA-SWNTs have a height of 1.45 (±0.10) nm(FIG. 13). Temperature and duration of the heat treatment has apronounced effect on the chemical processes that occur in molten urea,and the present inventors have found that these factors also affect theyield of soluble SWNTs. The best results are achieved by heating themixture of pre-purified SWNTs and urea for 5-10 minutes at 150-160° C.When the reaction includes an aldehyde, e.g. p-anisaldehyde, the ureamelt process is interrupted after 1-5, preferably 1-2 minutes foraddition of the aldehyde before heating is continued. SWNTs modified bymolten urea alone (hereafter referred to as U-SWNTs) are highly solublein water (up to 10 g L⁻¹), but they are insoluble in common organicsolvents. Adding an aldehyde, in particular p-anisaldehyde to the meltyields SWNTs (nanotubes undergoing this treatment are hereafter referredto as UA-SWNTs) that are appreciably soluble in both water and methanol(up to 1 g L⁻¹) and also results in approximately a ten-fold increase inthe yield of soluble nanotubes. The concentration (mg L⁻¹) of SWNTs inextracts of the product is estimated by dividing the absorbance (per cmpath length) at 500 nm by the factor 0.0286. J. L. Bahr, E. T.Mickelson, M. J. Bronikowski, R. E. Smalley, J. M. Tour, Chem. Commun.2001, 193 (2001). Comparing the sum of SWNT mass in each extract (wateror methanol) to the mass of RFP-SWNT used in the reaction provides anestimate of the yield of soluble SWNTs. Examples are provided furtherbelow. In the U-SWNT preparation given in Example 9, the ratio of themass of U-SWNT to the mass of RFP-SWNT (12.6 mg) is 0.034. In the twoUA-SWNT preparations also given in Example 9, the ratios are 0.36 (2.7mg RFP-SWNT) and 0.31 (34 mg RFP-SWNT). Without wishing to be bound byany theory, the 10-fold enhancement in yield when p-anisaldehyde isadded may be due to a “capping” effect, preventing the additionreactions of HNCO from reversing. In either case, the soluble SWNTs areisolated from excess urea and p-anisaldehyde by extraction,precipitation with sodium perchlorate, gel filtration, or a combinationof these methods before analysis.

Besides good solubility in water and methanol, a remarkable feature ofthe SWTNs prepared according to the present invention, in particularUA-SWNTs is their tendency to self-organize into crystal-like structuresupon evaporation from alcoholic solutions, e.g. methanol solutions.

In the following reference is made to the figures, wherein

FIG. 1: shows UV-visible absorption spectra of RFP-SWNTs treated withmolten urea and then dissolved in water, before (dashed curve) and after(continous curve) removal of components with high affinity forcross-linked dextran (Sephadex® G-100).

FIG. 2: shows a tapping mode AFM image from a solution of urea-treatedRFP-SWNTs applied to a film of polystyrene on mica. The solution usedwas the one whose UV-visible spectrum is shown in FIG. 1 (continuouscurve) and was applied to the film by a spin-coating process.

FIG. 3: shows the postulated chemical reactions involved in themodification of CNTs by treatment with molten urea. The CNTs werepreviously treated to generate oxidized carbon groups, mainly atnanotube ends and sidewall defects. The three oxidized carbon groupsimplicated are carboxylic acid, phenol, and epoxide, but other groupscould also be involved.

FIG. 4: shows UV-visible absorption spectra of RFP-SWNTs treated withmolten urea and then dissolved in water according to Example 1 (seebelow), after fractionation by column chromatography over Sephadex®G-100 into Solution BI and Solution B2 according to Example 4. SolutionB1 was diluted by a factor of 2.5 to obtain the spectrum shown.

FIG. 5: shows emission spectra of Solution B1 (diluted by a factor of2.5) and Solution B2 from Example 4 (see below) when excited with 410-nmlight. The absorption spectra of these samples are shown in FIG. 4.

FIG. 6: shows UV-visible absorption spectra of RFP-SWNTs treated withmolten urea and then dissolved in water according to Example 5 (seebelow), before and after fractionation by column chromatography overSephadex® G-10 according to Example 6 (see below). Solution C (beforefractionation) was diluted by a factor of 100 to obtain the spectrumshown (dashed curve). Solution D1 (first fraction) was diluted by afactor of 25 to obtain the spectrum shown (continuous curve). Solution Ewas obtained from the second fraction by precipitation with sodiumperchlorate and re-dissolution in water (dash-dotted curve).

FIG. 7: shows emission spectra of Solution Dl (diluted by a factor of25) and Solution E from Example 6 and Example 7 (see below),respectively, when excited with 410-nm light. The absorption spectra ofthese samples are shown in FIG. 6. The signal from a solution of wateralone is also shown (dotted curve).

FIG. 8: shows TEM images of UA-SWNT crystalline structures that formwhen solutions of UA-SWNTs in methanol (ca 800 mg L⁻¹) are allowed toevaporate on holey carbon films. Scale bars: A, 1 μm; B, 1 μm; C, 5 nm.

FIG. 9: shows SEM images of UA-SWNT crystalline structures that formwhen solutions of UA-SWNTs in methanol (ca 800 mg L⁻¹) are allowed toevaporate on silicon substrates (A and B, 400-nm oxide; C, nativeoxide). In A, some of the plate-like objects extend over the edge of thesubstrate (on right). In B, the light-colored structure containingnanotubes is partially submerged in a grainy material that probablycontains metal catalyst nanoparticles. Scale bars: A, 1 μm; B, 3 μm; C,10 μm.

FIG. 10: shows polarized optical microscope images of UA-SWNTcrystalline structures taken in reflection mode using white incidentlight and a pair of crossed polarizer (0°) and analyzer (90°), with thecrystal orientation of 0° or 45°. The crystalline structures do notnoticeably polarise unpolarised light. Scale bars: 20 μm.

FIG. 11: shows IR absorption spectra of U-SWNTs and UA-SWNTs on siliconsubstrates, obtained with an IR microscope in transmission mode. Forcomparison, the spectra of urea and p-anisaldehyde are also shown Thesamples are prepared by allowing solutions in methanol to evaporate onnative oxide silicon or by applying neat p-anisaldehyde tohydrogen-terminated silicon.

The invention will now be further described and better understood by thefollowing examples which are presented for illustrative, non-limitingpurposes.

EXAMPLE 1 Characterization

In all examples that follow, one or several of the followingcharacterization steps were performed: UV-visible absorption spectrawere measured with a Varian Cary 50 spectrophotometer, using 1 cmpath-length quartz cuvettes. Tapping mode AFM measurements were made inair using a Digital Instruments Dimension 3100 scanning probe microscopewith a NanoScope IV Controller and tetrahedral tip silicon cantilevers(Olympus Optical OMCL-AC160TS). The TEM measurements were performedusing a TECNAI G2 F20 instrument operated at 200 kV. The samples for theTEM experiments were prepared using holey carbon films on 300-meshcopper grids (Plano S147-3). For SEM imaging, a Leo Gemini 1530 FieldEmission Scanning Electron Microscope was used with beam voltage of 5kV. For optical microscopy, we used a Leica DMRX polarizing microscope,in reflectance mode. The illumination source was a 12V, 100W halogenbulb. The images were captured using a Hamamatsu C4742 cameracoupled-with a Micro-Color RGB-MS-C color filter. IR absorption spectrawere obtained with a Bruker ISF 66 FT-IR spectrometer with an IRscope IIattachment. The spectra were recorded in transmission mode with aresolution of 10 cm⁻¹.

Modification of CNTs Using Urea Melt

The CNTs used for these experiments were purchased from CarbonSolutions, Inc. (5094 Victoria Hill Drive, Riverside, Calif. 92506). Theproduct name is RFP-SWNT. According to information provided by thesupplier, RFP-SWNT is prepared from their product named AP-SWNT by acidpurification with subsequent processing to reduce functionality. TheAP-SWNT product consists of single-walled CNTs prepared by the electricarc method. Hu et al. (2001)⁴⁷ determined. the mole percentage of acidsites (including carboxylic acids, lactones, and phenols) relative tothe total amount of carbon in RFP-SWNTs by acid-base titration and found1%, of which 0.7-0.8% could be attributed to carboxylic acid groups. Thepercentages found for the CNTs without the processing step to reducefunctionality were 2.7% (total acid sites) and 2.1-2.2% (carboxylic acidgroups). Thus it can be concluded that the process used to reduce thefunctionality of acid purified AP-SWNT CNTs reduces it by approximatelytwo-thirds. The degree of functionality on RFP-SWNT is stillappreciable, however, approximately one carbon per 100 being bonded toat least one oxygen atom to produce an acidic site, approximatelythree-quarters of which are —COOH groups. These groups are expected tobe located predominantly at nanotube ends and sidewall defect sites.

RFP-SWNT (2.46 mg) was placed in a glass test tube (Schott Duran®,12×100 mm) and pulverized with a glass stirring-rod (5 mm diameter).Urea (98.5 mg) was added and thoroughly pulverized with the RFP-SWNT toyield a gray powder. The powder was heated by placing the bottom of thetest tube at the exit port of a heat gun (Steinel Typ 3449, 2000 W) setto have a maximum temperature at the exit of 160° C. The powder meltedto give a black liquid, which was kept agitated by rotating and shakingthe tube manually. After 10 minutes, the mixture was allowed to cool toroom temperature, yielding a black solid. Water (0.50 ml) was added tothe solid, which appeared to dissolve completely within a few seconds bysimple mixing, giving a dark black solution. A small volume (10 μl) ofthe solution was diluted with water (1000 μl) in a quartz cuvette andthe UV-visible absorption spectrum was recorded. The spectrum is shownin FIG. 1 (dashed curve). The solution in the test tube was transferredto a small polypropylene centrifuge tube together with the solution inthe cuvette. It was centrifuged at 5000 rpm for 10 minutes two times.The pH of the solution was 8.15.

EXAMPLE 2 Chromatography Over Sephadex G-100

Two drops (approximately 50 μl) of the centrifuged solution from Example1 were applied to a small column (7×45 mm) of Sephadex® G-100 swollen inwater. After entering the column bed, the solution was eluted withwater. A gray band moved through the column and was collected. Anothergray band remained at the top (1-2 mm) of the column bed and could notbe washed out The solution that was collected was transferred to aquartz cuvette, diluted with water (total volume approximately 0.7 ml),and the UV-visible absorption spectrum was recorded. The spectrum isshown in FIG. 1 (continuous curve). It shows a nearly featureless risein absorption from 1000 nm to 300 nm and a maximum at 242 nm. Thissolution is referred to below as Solution A.

EXAMPLE 3 AFM Measurements

A substrate for AFM measurements was prepared by applying a drop of a2.5 wt-% solution of polystyrene (Aldrich #44.114-7. average M_(w) ca.350.000) in toluene to a mica substrate. The substrate was rotated at600 rpm while a drop (20 μl) of Solution A (undiluted) was applied tothe center. The rotation rate was increased to 700 rpm, causing most ofthe solution to be ejected from the substrate. Afterwards, spinning itat 4000 rpm for 90 s dried the substrate. An AFM image obtained byscanning at a location near the center of the substrate is shown in FIG.2, together with a zoomed-in section of the image.

The fact that the RFP-SWNT material becomes water-soluble after meltingits powdered mixture with urea is most likely due to covalentmodification of the CNTs or non-covalent adsorption of water-solublepolymer products. While not intending to be limited to any particulartheory, the present inventors favor the covalent modificationpossibility due to the fact that the same process fails to renderas-prepared single-walled CNTs water-soluble. As noted above, roughlyone carbon atom per 100 of the CNTs in RFP-SWNT is in a chemicallyoxidized acidic state, based on the results of Bower et al. ((1998)Chem. Phys. Lett. 288, 481). Of the functional groups responsible forsuch sites, —COOH and phenolic —OH groups are reactive with isocyanates,including isocyanic acid. The reaction of isocyanic acid with thesegroups and subsequent addition reactions to generate polyisocyanateappendages are shown schematically in FIG. 3. High molecular weightpolyisocyanates were first reported in 1959 and represent the simplesttype of nylon (nylon-1) (Bur et al. (1976) Chem Rev. 76, 727). Anotherpossible reaction shown in FIG. 3 is the ring opening of epoxide groupsby ammonia to generate amine and alcohol groups, which then react withisocyanic acid. Lu et al. (2002) (J. Phys. Chem. B 106, 2136) recentlyproposed using the ring-opening step as a way to functionalize thesidewalls of single-walled CNTs. It should be noted that polymer chainbranching and cross-linking reactions are also possible, since theH-atoms bonded to the N-atoms of the linear chains are expected to bereactive with isocyanic acid.

The growth of polyisocyanate on the RFP-SWNTs could account for thewater-solubility of the product as well as the dissociation of nanotubebundles into individual tubes, as is indicated by the AFM images. Thediscrete round-shaped bodies seen attached in a pearl chain-like mannerto the CNTs in the AFM images are presumably the polymers. The sizes ofthe bodies of several nanometers suggest polymer molecular weights ofthe order of 10,000.

Since the CNTs in the RFP-SWNT material are expected to occur inbundles, the growth of polyisocyanate uniformly along the nanotubes, asthe AFM results suggest, indicates that dissociation of the bundlesoccurred at an early stage of the reaction This surprising resultimplicates the tendency of urea to form inclusion compounds. Althoughthe channel in the usual inclusion compounds between urea andhydrocarbons are much too small to accommodate CNTs, the transientformation of larger cages should be considered. Urea is also widely usedfor denaturing proteins, which involves the disruption of non-covalentbonds within proteins, including van der Waals interactions. Since vander Waals interactions are important for the bundling of CNTs, theability of urea to cause dissociation may be related to its ability tode-nature proteins.

EXAMPLE 4 Chromatography Over Sephadex® G-100

A 250-μL aliquot of the centrifuged solution from Example 1 was appliedto a small column (10×80 mm) of Sephadex® G-100 swollen in water. Afterentering the column bed, the solution was eluted with water. Once theeluting solution became colored (black-brown), 1.1 mL was collected:this solution is referred to below as Solution B1. The next 1.5 mL(yellow colored) was also collected and is referred to as Solution B2. Ablack-brown band remained at the top (˜5 mm) of the column and could notbe washed out with water. The UV-visible absorption spectra of these twosolutions are shown in FIG. 4. The spectrum of Solution B1 (afterdilution with water by a factor of 2.5) is characterized by a maximum at235 nm and a tailing absorption to beyond 1000 mm (FIG. 4, continuouscurve), weak but distinct inflections occur in the wavelength region400-500 nm. The spectrum of Solution B2 (FIG. 4, dashed curve) ischaracterized by a slightly structured absorption in the wavelengthregion 400-500 nm and almost no absorption beyond 700 nm. There is noobvious maximum in the UV region. Despite this difference in absorptionspectral characteristics, both solutions have a yellow emission with amaximum near 515 nm (FIG. 5). The emission from Solution B2 is 4 timesmore intense than that from B1, despite that fact that the absorbance ofB1 is 4.5 times greater than the absorbance of B2 at the excitationwavelength (410 nm). Thus it can be concluded that two (or more)components contribute to the absorption and emission properties ofSolution B1. One contribution is due to the characteristics offunctionalized CNTs (plasmon absorption maximum below 250 nm,structure-less absorption tail into the near infrared, and little or noemission). The other contribution is due to a component that is alsopresent in Solution B2 with a yellow emission maximum 515 nm. Thiscomponent is likely to be responsible for absorption between 400 nm and500 nm in both solutions. It is a product of the reaction between theRFP-SWNT and molten urea and appears to be the component that makes theCNTs soluble in water.

EXAMPLE 5 Modification of CNTs Using Urea Melt

RFP-SWNT (12.6 mg) was pulverized with urea (504 mg) in a glass testtube as described in Example 1. The powdered mixture was heated with aheat gun source as in Example 1 for 7.5 minutes, cooled, mixed withwater (800 μL) water, and centrifuged (5000 rpm for 10 min). Theresulting solution, referred to below as Solution C, had a pH of 9.6. Asmall volume (10 μl) of the solution was diluted with water (1000 μl) ina quartz cuvette and the UV-visible absorption spectrum was recorded.The spectrum, shown in FIG. 6 (dashed curve), has a tailing absorptionto beyond 1000 nm. No maximum is apparent in the UV region due toabsorption by urea and by-products of the reaction.

EXAMPLE 6 Chromatography Over Sephadex® G-10

A 500-μL aliquot of the centrifuged solution from Example 5 was appliedto a small column (10×52 mm) of Sephadex® G-10 swollen in water. Afterentering the column bed, the solution was eluted with water. Once theeluting solution became colored (black-brown), 1.0 mL was collected:this solution is referred to below as Solution D1. The next 0.9 mL(black-brown colored) was also collected and is referred to as SolutionD2. A light gray band remained at the top (˜11 mm) of the column andcould not be washed out with water. The UV-visible absorption spectrumof Solution D1 (after dilution with water by a factor of 25) ischaracterized by a maximum at 220 nm and a tailing absorption to beyond1000 nm (FIG. 6, continuous curve). The diluted Solution D1 ischaracterized by a yellow emission with a maximum near 515 nm (FIG. 7,continuous curve).

EXAMPLE 7 Isolation of Modified CNTs by Precipitation with SodiumPerchlorate

Sodium perchlorate monohydrate (115 mg) was added to Solution D2 fromExample 6, and then mixed until the crystals dissolved, giving aconcentration of NaClO₄ of ˜1.0 M. The solution became opalescent withinminutes. After being left overnight, a black precipitate settled outleaving clear yellow-brown supernatant, which emitted yellow-white lightwhen excited with UV (366 nm) light. After removing the supernatant, theprecipitate was readily redissolved in 500 μL of water (Solution E). TheUV-visible absorption spectrum of Solution E (after dilution with waterby a factor of 20) is characterized by a maximum at 242 nm and a tailingabsorption to beyond 1000 nm (FIG. 6, dash-dotted curve). The dilutedsolution is also characterized by a weak yellow emission whose maximumis near 550 nm (FIG. 7, dash-dotted curve). Sodium perchloratemonohydrate (115 mg) was added to Solution E, and then mixed until thecrystals dissolved, giving a concentration of NaClO₄ of ˜0.25 M. Thesolution became opalescent within minutes. After 30 minutes, it wascentrifuged at 5000 rpm for 10 minutes, yielding a black precipitateand, clear colorless supernatant. After removing the supernatant, theprecipitate was dried under a gentle stream of compressed air. Theresulting solid, having a mass of 0.47 mg, redissolved in 10 μL of waterafter bath sonication for 1 minute.

EXAMPLE 8 Preparation of SWNTs Modified by Molten Urea Alone (HereinReferred to as “U-SWNTs”.)

A portion of RFP-SWNT (2.51 mg) was placed in a glass test tube (SchottDuran®, 12×100 mm) and pulverized with a glass stirring-rod (5 mmdiameter). Urea (100 mg) was added and thoroughly pulverized with theRFP-SWNT to yield a gray powder. The powder was heated by clamping thetest tube vertically over the exit port of a heat gun (Steinel Type3458, 2000 W) that was set to have a maximum temperature at the exit of150° C. The powder melted to give a black liquid, which was keptagitated by hand using a stirring-rod. After 8 minutes of heating, themixture was cooled to room temperature. The soluble material wasextracted into water (600 μl) by vortex mixing, and the resultantsuspension was centrifuged at 5000 rpm for 10 minutes to give a clear,mildly basic (pH 9.6) supernatant A portion (45 μl) of the supernatantsolution was diluted with water (1000 μl) in order to measure theUV-visible absorption spectrum for estimation of the concentration.Based on the absorbance at 800 nm (0.093) and the dilution factor, theconcentration of nanotubes in the supernatant was approximately 110 mgl⁻¹. The estimated mass of nanotubes in the supernatant was thusapproximately 0.07 mg, or roughly 3% of the mass of RFP-SWNT used in thereaction. The U-SWNT was precipitated from the remaining supernatant,after removing the undissolved material, by adding sodium perchloratemonohydrate (20.8 mg). Centrifugation (3000 rpm for 5 minutes) gave ablack precipitate and clear yellow supernatant. The UW-visibleabsorption spectrum of the supernatant, after dilution, exhibits somestructure and tails to around 800 nm. The supernatant solutions have ayellow-green emission under 366-nm light. The precipitate dissolvedreadily in water. This preparation is summarized in Table 2.

Preparation of SWNTs Modified by Molten Urea with Addition of anAldehyde, e.g. p-anisaldehyde (Herein Referred to as “UA-SWNTs”.)

Two preparations of UA-SWNT are summarized in Table 2. One preparationwas on a scale similar to that of the U-SWNT preparation described aboveand the other preparation was scaled up approximately ten-fold. In thelatter case, an agate mortar and pestle was used to grind the RFP-SWNTand urea, and a larger glass test tube (Schott Duran®, 17×170 mm) wasused as the reaction vessel. In both cases, the mixture of RFP-SWNT andurea was heated to approximately 150° C. for 1-2 minutes until it hadthoroughly liquefied. For the smaller scale reaction, heating was for 1minute. For the larger scale reaction, 2 minutes heating was requireduntil the mixture was completely molten. Then the heating was stopped,p-anisaldehyde was added, and heating was continued for an additional 7minutes. The resultant solid, after cooling to room temperature, wassubjected to multiple extractions, first with water and later withmethanol. Each extraction step typically involved vortex mixing, atleast 30 minutes wait, and brief (1 minute) bath ultrasonication beforecentrifuging the suspension (5000 rpm for 10 min). The extractions withwater removed the excess urea and other water-soluble components,including ca 20-30% of the excess p-anisaldehyde, whose solubility inwater is ca 0.015 M. The extracts also contained the yellow coloredcomponent noted above. As indicated in Table 2, the concentration ofnanotubes. in the initial extract(s) with methanol was relatively low.The concentrations of subsequent extracts increased and then decreased,presumably due to depletion of soluble material from the remainingsolid. The UA-SWNTs in the methanol extracts of the larger scalepreparation were isolated by precipitation with sodium perchloratemonohydrate (4-9 mg per ml). The precipitates were dissolved in methanolafter washing. These samples were apparently much more sensitive to saltconcentration than those of U-SWNT in water because the precipitates didnot dissolve the first time they were suspended in methanol (1 ml).After centrifuging and removing the supernatant, the precipitates weremostly soluble in methanol (1 ml). Apparently, it was necessary toremove traces of sodium perchlorate with methanol before the nanotubesbecame soluble. The concentration of nanotubes in the next to the lastmethanol extract (step 11 in table 2) was 980 mg l⁻¹, indicatingsolubility up to 1 g l⁻¹ for UA-SWNTs in methanol. These nanotubes arealso quite soluble in water.

Estimation of the solubility of U-SWNTs in water. U-SWNTs were preparedas described above. The aqueous extract (0.5 ml) was applied to a column(10×52 mm) of Sephadex G-10 and eluted with water. The first 1.0 mlcontaining nanotubes that eluted from the column was collected anddetermined by UV-visible absorption to contain 100 mg l⁻¹ U-SWNTs. Thesolution was evaporated to dryness with compressed dry air. Theresulting solid dissolved within seconds upon addition of a 10-μl dropof water, indicating solubility up to 10 g l⁻¹. The solution is toodarkly colored to see whether the solid was completely dissolved, but nosolid was deposited onto the wall of the container when the solution wasshaken vigorously.

TABLE 2 Summary of preparations of U-SWNT and UA-SWNT. Total Vol., Conc.mass RFP- Vol., Conc. MeOH ex- SWNT Urea Aldehyde H₂O extracts extractstracted Yield (mg) (mg) (μl) (ml, mg l⁻¹) (ml, mg l⁻¹) (mg) (%) 2.51 1000.6, 110 0.07 3 2.74 110 20 1) 1.0, <1  5) 1.0, 40 0.94 34 2) 0.5, <1 6) 1.1, 380 3) 0.5, <2  7) 1.0, 210 4) 0.5, 10  8) 1.0, 160  9) 1.0, 6010) 1.0, 50 34.0 1370 250 1) 1.5, 10  7) 4.0, 30 10.5 31 2) 9.0, 10  8)4.0, 10 3) 3.0, 70  9) 4.5, 720 4) 3.0, 190 10) 4.4, 710 5) 3.0, 100 11)4.4, 460 6) 3.0, 130 12) 4.1, 100

Without wishing to be bound by any theory, the scenario that isenvisaged by the present inventors is the intercalation of the CNTbundles by urea, urea decomposition, addition reactions between thedecomposition products and oxidized carbon groups on the CNTs, andpolymer growth from those sites. These processes may begin with themixing, e.g. the grinding together of the CNTs and urea, which may alsoresult in mechanical cutting of the CNTs.

The procedure described in this report of invention has several of thecharacteristics that were listed above for the ideal procedure:

-   -   1. It is relatively simple and fast    -   2. It yields isolated CNTs that dissolve in water.    -   3. It is applicable for CNTs that have not been shortened.    -   4. It provides soluble CNTs with functional groups for        subsequent chemical modification.

Further, the procedure is applicable for multi-walled CNTs. It is alsoclear that suitable chemical modifications, introduced either during thereaction or afterwards, are possible for making the CNTs soluble in avariety of organic solvents. It may be impossible to perform suchreactions without altering the electronic and/or mechanical propertiesof the CNTs, but it may be possible to restore them finally by thermaltreatment under vacuum or inert atmosphere. None of the state of the artprocedures offer this combination of features.

The features disclosed in the foregoing description and the claims may,both separately and in any combination thereof be material for realizingthe invention in diverse forms thereof.

LIST OF REFERENCES CITED IN TABLE 1

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²Bandow, S.; Rao, A. M.; Williams, K. A.; Thess, A. Smalley, R E.,Eklund, P. C. (1997) J. Phys. Chem. B 101, 8839.

³Dujardin, E.; Ebbesen, T. W.; Krishnan, A.; Treacy, M. M. J. (1998)Adv. Mater. 10, 611.

⁴Liu, J.; Rinzler, A. G.; Dai, H.; Hafner, J. H.; Kelley Bradley, R.;Boul, P. J.; Lu, A.; Iverson, T.; Shelimov, K.; Huffman, C. B.;Rodriguez-Macias, F.; Shon, Y.-S.; Lee, T. R.; Colbert, D. T.; Smalley,R. E. (1998) Science 280, 1253.

⁵Zimmerman, J. L; Kelley Bradley, R.; Huffman, C. B.; Hauge, R. H.;Margrave, J. L. (2000) Chem. Mater. 12, 1361.

⁶Tang, X.-P.; Kleinhammes, A.; Shimoda, H.; Fleming, L; Bennoune, K. Y.;Sinha, S.; Bower, C.; Zhou, O.; Wu, Y. (2000) Science 288, 492.

⁷Rao, C. N.; Govindaraj, A. (2001) Proc. Indian Acad. Sci. (Chem. Sci.)113, 375.

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1. A carbon nanotube, produced by a) providing, in any order: carbonnanotubes, and urea as a precursor of isocyanic acid and/or cyanate ion,said isocyanic acid and/or cyanate ion being capable of undergoing apolymerization reaction; b) mixing together the nanotubes with urea; c)initiating a polymerization reaction of the isocyanic acid and/orcyanate ion to yield modified carbon nanotubes, wherein the carbonnanotubes have functional groups on their surface and/or ends, andwherein said polymerization reaction occurs at said functional groups,wherein, before, during or after the polymerization reaction, at leastone aldehyde is added to the mixture, further comprising: heating themixture, or alternatively, acidifying the mixture, or both heating andacidifying the mixture, wherein the at least one aldehyde is addedduring the heating and/or acidification.
 2. The carbon nanotubeaccording to claim 1, wherein the nanotube is non-bundled.
 3. The carbonnanotube according to claim 2, wherein it is decorated in a pearlchain-like manner with discrete bodies, when viewed by AFM.
 4. Thecarbon nanotube according to claim 1, having one or more physicalcharacteristics selected from the group consisting of: a solubility inwater or aqueous solution of up to approximately 10 g/l and/or asolubility in methanol or methanolic solution of up to approximately 1g/l, an absorption maximum between 210 nm and 250 nm, and one or severalor all of the following absorption maxima in the infrared in thefollowing wavenumber regions: 3470-3490 cm⁻¹, 3420-3440 cm⁻¹, 3365-3385cm⁻¹, 3330-3350 cm⁻¹, 3245-3265 cm⁻¹, 3210-3230 cm⁻¹, 1660-1680 cm⁻¹,1610-1630 cm⁻¹, 1450-1470 cm⁻¹, 1330-1350 cm⁻¹ and 1095-1115 cm⁻¹.
 5. Anassociation of carbon nanotubes according to claim 3, wherein the carbonnanotubes are non-bundled, but interconnected at their ends through saiddiscrete bodies into branched structures, when viewed by AFM.
 6. Acarbon nanotube or an association of carbon nanotubes produced by a)providing, in any order: carbon nanotubes, and urea; b) mixing togetherthe nanotubes and the urea; c) heating the mixture of b) wherein theheating is for approximately 1-60 min, wherein the product of step c) isdissolved in aqueous solution or alcoholic, preferably methanolicsolution or a mixture of an aqueous and an alcoholic solution, andsubjected to a salt precipitation step, and/or centrifugation orfiltration, each of both possibilities optionally followed by washing,and/or subjected to size separation and/or adsorption and/or enzymaticdegradation, and/or selective burning, and/or plasma treatment, andwherein the product of step c) is dissolved in aqueous solution oralcoholic, preferably methanolic solution or a mixture of an aqueous andan alcoholic solution, and is subjected to an evaporation step.
 7. Thecarbon nanotube or association of carbon nanotubes according to claim 6,which is at least partially crystalline.
 8. The carbon nanotube orassociation of carbon nanotubes according to claim 6, having one or morephysical characteristics selected from the group consisting of: one orseveral or all of the following absorption maxima in the infrared in thefollowing wavenumber regions: 3370-3390 cm⁻¹, 3205-3225 cm⁻¹, 3060-3080cm⁻¹, 1700-1720 cm⁻¹, 1680-1700 cm⁻¹, 1655-1675 cm⁻¹, 1580-1600 cm⁻¹,1500-1520 cm⁻¹, 1440-1460 cm⁻¹, 1395-1415 cm⁻¹, 1245-1265 cm⁻¹,1155-1175 cm⁻¹, 1020-1040 cm⁻¹, 845-865 cm⁻¹, 795-815 cm⁻¹, having bothcrystalline and amorphous parts, when viewed under AFM, TEM and/or SEM,and a solubility in water or aqueous solution of up to approximately 10g/l and/or a solubility in methanol or methanolic solution of up toapproximately 1 g/l.
 9. A method of conducting electricity comprisingdeploying a carbon nanotube according to claim 1 in an electronicdevice, a nanoelectronic device, a memory element, a field emissiondevice, a sensor, an actuator, an electromechanical device, a compositematerial, a coating/paint/paste, a hydrogen storage device, a battery orfuel cell, a supercapacitor, a photoelectrochemical device, aphotovoltaic device, an energy conversion device, a light emittingdiode, a liquid crystal display, a probe scanning probe microscopy, anon-linear optical device or antenna, or a catalyst.
 10. A method ofabsorbing gas molecules comprising deploying a carbon nanotube accordingto claim 1 in an electronic device, a nanoelectronic device, a memoryelement, a field emission device, a sensor, an actuator, anelectromechanical device, a composite material, a coating/paint/paste, ahydrogen storage device, a battery or fuel cell, a supercapacitor, aphotoelectrochemical device, a photovoltaic device, an energy conversiondevice, a light emitting diode, a liquid crystal display, a probescanning probe microscopy, a non-linear optical device or antenna, or acatalyst.